1. Field of the Invention
The present invention relates to converting wood pulping process black liquor soap skimmings to crude tall oil (CTO) and refining the crude tall oil for separation of the tall oil fatty acids and tall oil resin acids contained therein. More particularly, the present invention relates to improvements in tall oil refining to avoid fouling the refinery heads loop coolers and causing a significant loss in heat transfer.
2. Description of Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98)
It has long been appreciated that the black liquor residue from wood pulping contains valuable chemicals, which make up the CTO and have various industrial applications. The black liquor contains the soaps of rosin and fatty acids, as well as sodium lignate and the spent cooking chemicals for reuse. On concentration of the spent pulping liquor, the sodium soap of these mixed acids rise to the surface and can be skimmed off. This material is referred to as xe2x80x9csoap skimmingsxe2x80x9d or xe2x80x9ctall oil soap.xe2x80x9d The soap skimmings are converted to CTO by reaction typically with sulfuric acid and then separated from the simultaneously formed spent acid by batch cooking, continuous centrifuging, or continuous decanting. The CTO is normally divided into various fractions by distillation, which first extracts the pitch fraction. The de-pitched CTO is then separated into fractions of heads, tall oil rosin (TOR), tall oil fatty acids (TOFA), and distilled tall oil (DTO).
In the period since Hutch Holton disclosed in U.S. Pat. No. 4,012,280 that anthraquinone can be beneficially employed as a pulping aid to increase pulp yield, the use of anthraquinone in this application has grown. While its use is beneficial to pulp yield, some of the chemical ends up in the black liquor soap skimmings and, consequently, in the CTO. When the CTO is refined, the anthraquinone will condense in the upper cooling zones of the refinery towers, as well as in external heat exchangers and in the vacuum system. Such condensation reduces the efficiency of the refining operation by loss in heat transfer capability and eventually requires cleaning of the towers.
One solution proposed for this problem is described in commonly-owned pending U.S. patent application Ser. No. 09/390,153, now abandoned. A means was disclosed for modifying the conventional tall oil refinery to avoid fouling of the cooling zones of the towers and associated heat exchangers by employing a warm water loop for cooling the tall oil heads fraction extracted from the crude tall oil, rather than using cold water. This (""153) solution reduced or eliminated the anthraquinone condensation and subsequent solidification and crystallization in heads loop direct contact coolers and associated heat exchangers. Unfortunately, the problem with anthraquinone condensation in the tall oil distillation process is more pervasive, and said solution provides only limited benefits.
In view of the various types of condensation equipment available, including indirect contact type coolers, as well as the various temperature ranges existing in the tall oil distillation columns themselves, there remains an anthraquinone condensation and solidification problem unsolved. Therefore, a further means is needed to deal with the anthraquinone-induced equipment fouling associated with tall oil distillation.
The object of the invention is met in the refining plant flow diagram disclosed herein. In particular, the invention provides a process for avoiding anthraquinone condensation and solidification in a tall oil distillation column with an indirect contact condenser connected in series with the column. A warm-water loop is provided for the tube-side flow of tempered water to prevent anthraquinone from fouling the outer surfaces of the exchanger tubes and causing a loss in heat transfer and increased pressure drop.